1. Field of the Invention
The present invention relates to easy-opening can ends and more particularly to an improved tab of increased longitudinal strength for use as a retained tab on an improved ecology-type can end.
2. The Prior Art
It is known in the art to provide container end wall structures which include a retained tab and a retained tear strip so that in use, no container component is separated to form litter. These type containers and end structures have been referred to as ecology ends.
Typical of one form of ecology ends are those shown and described in U.S. Pat. Nos. 4,024,981 and 4,084,410 of May 24, 1977, respectively and Apr. 10, 1979, assigned to the same assignee.
U.S. Pat. No. 4,024,981 describes an improved longitudinally rigid tab and an improved end structure wherein the tab initially acts as a class 2 lever to initiate fracture of the portion of the scoreline immediately in front of the rivet. Thereafter the tab operates as a class 1 lever to complete fracture of the scoreline and to push the retained pour panel downwardly, the panel being retained on the end wall.
U.S. Pat. No. 4,084,410 relates to a tab with increased resistance to removal by bending of the tab through the provision of a plurality of bending regions.
Regardless of the details of the structure of the tab, those tabs intended to be used in ecology-type ends require substantial longitudinal rigidity especially if the tab is intended to function as a class 2 and then a class 1 lever in an opening sequence.
In the tabs described in the above patents, longitudinal rigidity was achieved by curling the side legs of the tab to provide a double thickness of metal extending essentially from the front to the rear of the tab. Additionally the front end of the tab is formed of multiple layers of sheet material, preferably of one piece.
Initially, tabs as described were manufactured from aluminum alloy sheet stock of 0.018 inch thickness, and operated satisfactorily. Such tabs included an intermediate web of material at the front end of the tab, the web including space finger elements extending rearwardly and disposed within the folds of the side legs and terminating approximately at the forward end of the aperture in the flap and through which the rivet extends to attach the tab to the end wall. This structure was found to be of sufficient longitudinal strength when the tab was fabricated of sheet stock of 0.018 inch thickness. The price of aluminum has, however, increased significally and thus, there has been a tendency to use thinner stock, i.e. 0.016 inch thick material, in the fabrication of tabs. The result has been that under certain circumstances there have been tab failures where the tabs have been fabricated of lighter stock in order to save in material costs.
For example, some beverages are packed under substantial pressure or exposed to substantial pressure during the packing operation. In such cases the scoreline normally includes a relatively high residual, i.e. the dimension of the unscored metal beneath the score area is comparatively high. The result was that in some instances, tabs of the lighter stock, i.e. 0.016 inch material, failed by bending in a zone in front of the rivet and to the rear of the front of the tab. The observed mode of failure appeared to be the pulling of the fingers out from under the curls along the side legs, or actual bending of the side legs, or both. In other instances tab failure appeared to result from the fact that the tab was misaligned, i.e. rotated about the rivet with the result that a portion of the nose of the tab was over the end wall rather than being over the pour panel, with the result that tabs of the thinner stock exhibited a tending to fail by bending, as described.
It has also been noted, in some instances that the scoreline in front of the rivet was prematurely rupturing under certain circumstances. More particularly, the tab is attached to the end wall by a rivet which passes through an aperture in the ear of the tab, the ear being located to the rear of the nose or opening end of the tab, and the scoreline including a portion immediately in front of the rivet. In some instances the end wall of the packed container is slightly bulged due to pressure developed during packing or because of the internal pressure of the container, or both.
In such cases, it is believed that the entire tab is raised a very small distance above the end wall, i.e. there is a small vertical clearance between the underside of the tab and the end wall. Normally such a clearance presents no problem, except it has been observed in some instances that there was premature fracture of the portion of the scoreline to the front of the rivet. It is believed that such premature fractures, when they occur, are caused by the normal vibration of the stacked containers during shipping.
It is believed that the vibration is transmitted to the tab and through the tab ear to the rivet with the result that the portion of the end wall around the rivet is stressed through the vibration, in a vertical direction, of the tab, the latter being free to move up and down because of the small clearance between the underside of the tab and the end wall.